The Slug Prince is encountered in the Witchaven Dungeon during The Slug Menacequest. He can only be fought once and requires 30 Slayer to kill. The people of Witchaven are obviously very mad at you for killing the Slug Prince, as after you kill him they tell you to leave them alone.

The Slug Prince can only be fought with melee; using Ranged or Magic is futile, as they bounce harmlessly off. However, he is very easy, and high and medium level players can kill him without prayer or food. If you do choose to use Protect from Melee, be aware that the Slug Prince's attacks may drain Prayer. A high or medium levelled player should have little to no problems fighting him, but lower levels might want to bring food or even prayer potions. A ring of recoil and a serpentine helm do not work against the Slug Prince.